A communication device for carrying out wireless/wired information communication is equipped with various radio-frequency components such as an amplifier, a mixer, and a filter. Among these components, a band pass filter (BPF) has a function to pass only a signal of a necessary specific frequency band (desired wave) by arranging a plurality of resonance elements.
In today's communication system, it is desired that a filter have a sharp cut-off characteristic and a low-loss characteristic in order to maximize available bandwidths in terms of efficient use of frequencies and elimination of interference. To realize these filter characteristics, it is necessary to mutually connect a plurality of resonance elements in an electromagnetic field. A circuit constant of the filter is determined based on the following parameters: 1: Resonance frequency fi of each resonance element, 2: Coupling coefficient Mij between resonators i and j, 3: External quality factor (Qe) coupling of an external circuit.
To improve steepness of a filter, it is necessary to increase the number of resonance elements, and to make the filter multi-pole; however, in general, insertion loss increases as the number of resonance elements increases, and thus, the insertion loss and the number of the resonance elements are in a trade-off relationship. Further, when there is structural restriction in configuring a multi-pole filter, there is a way to connect each resonance element by a transmission line such as a cable to make a filter multi-pole. Further, another way is to make groups configured from a plurality of resonance elements, and to connect the groups by a transmission line such as a cable to configure a filter.
However, in either case, loss of the cable or the like used for connection influences and deteriorates the filter characteristics. Especially, in a case of a superconducting filter, which has a low-loss characteristic, connection loss significantly deteriorates the filter characteristic. Therefore, a method for connecting resonance elements with low loss is desired.
Also, in wireless power transmission, which has been well-developed in recent years, power transmission is carried out by using and connecting a coil and resonance elements. However, connection amount decreases as a distance between the resonance elements increases, which lowers transfer efficiency.